Follow these steps:
Tools needed.....
1 . Custom firmware for iphone 3GS 4.2.1...Iphone custom firmware 4.2.1
2.Redsn0w
3. Apple original firmware 4.2.1 for 3GS - http://appldnld.appl....a_Restore.ipsw
1. Connect your phone with PC and switch off...
2. Open redsnow and choose Original firmware...
3. If you have newboot just press "yes"
4. now click "Next"
5. Choose "Just Enter pwned DFU mode right now"
6 now click "Next" again "Next" and follow the screen....
After that open itunes and press "Shift + Restore" and choose custom firmware 4.2.1...
Good Luck!
Courtesy of:
http://xsellize.com/topic/98192-how-to-upgrade-iphone-3gs-421-using-custom-firmwareguide/
It actually saved my mates iPhone after he failed to Jailbreak his 4.2.1 with the BB of 06:15 or 09:15, anyway, this happens when you selected the install ipad BB during the redsn0w installtion, this means you cannot restore the iPhone to standard firmware, and the failed jailbreak means your stuck in the recovery boot mode loop, programs such as iRecovery and smallumbrella don't work, this is the only method i found which worked. Hope i can help other, post as reply if this helped and also give credit to original postee on the website.
2. Redsnow - http://sites.google.....edirects=0&d=1
Tools needed.....
1 . Custom firmware for iphone 3GS 4.2.1...Iphone custom firmware 4.2.1
2.Redsn0w
3. Apple original firmware 4.2.1 for 3GS - http://appldnld.appl....a_Restore.ipsw
1. Connect your phone with PC and switch off...
2. Open redsnow and choose Original firmware...
3. If you have newboot just press "yes"
4. now click "Next"
5. Choose "Just Enter pwned DFU mode right now"
6 now click "Next" again "Next" and follow the screen....
After that open itunes and press "Shift + Restore" and choose custom firmware 4.2.1...
Good Luck!
Courtesy of:
http://xsellize.com/topic/98192-how-to-upgrade-iphone-3gs-421-using-custom-firmwareguide/
It actually saved my mates iPhone after he failed to Jailbreak his 4.2.1 with the BB of 06:15 or 09:15, anyway, this happens when you selected the install ipad BB during the redsn0w installtion, this means you cannot restore the iPhone to standard firmware, and the failed jailbreak means your stuck in the recovery boot mode loop, programs such as iRecovery and smallumbrella don't work, this is the only method i found which worked. Hope i can help other, post as reply if this helped and also give credit to original postee on the website.
2. Redsnow - http://sites.google.....edirects=0&d=1